In various areas of medicine, such as for example neurology, psychiatry or brain surgery, it is desirable to be able to localise specific functional areas of the brain in order to be able to map brain functions. If, for example, a brain tumour is to be removed by surgery, then as far as possible the tumour should be removed without, however, possibly damaging the so-called primary areas of the brain which play a decisive role in a person's motor and sensory systems, language, or visual capabilities. Surgery should, if possible, not damage these areas at all, or only to an exceedingly small extent.
According to a known direct method, such specific areas of the brain have been found intra-operatively by direct cortical stimulation (DCS) on an exposed cranium using electrodes. In this process, an electrode was inserted into a specific area of the brain and an electrical impulse applied, wherein the reaction of the person being examined following the electrical impulse, for example the twitching of a muscle or the perception of visual impressions, is observed. The specific areas of the brain located by direct cortical stimulation were marked using small, attached plates which helped the surgeon's orientation in a subsequent brain operation with respect to areas of the brain which are as far a possible not to be damaged. To date, direct cortical stimulation still represents the most precise method for mapping brain functions, enabling accuracy in the range of a few millimeters when locating specific areas of the brain. However, this method can only be performed intra-operatively, the person under examination being fully conscious. This, however, can lead to problems in the application of this method, since this is an unpleasant state for the person being examined, and if complications arise, the person cannot simply be laid down and made to relax, due to the exposed cranium.
Furthermore, various indirect methods are known for mapping brain functions, by which, however, only a considerably lower accuracy in locating specific areas of the brain may be achieved. Thus, in functional nuclear spin tomography (fMRI) for example, a person under examination has to perform specific actions, such as for example a sweep of the hand, which promotes blood flow to the areas of the brain assigned to these actions. This change in the blood flow in specific areas of the brain may be measured during neuronal activity due to the decoupling of blood flow and oxygen consumption, since this gives rise to hyperoxygenation and thus a drop in the concentration of paramagnetic deoxyhaemoglobin (BOLD-effect), which may then be measured by means of suitable sequences of nuclear spin tomography as a so-called “endogenic contrast medium”. However, as mentioned above, this method is relatively imprecise and only provides spatial resolution in the range 0.5 to 1.0 cm.
A method is known from Neurosurgery 1992–1998, December 1997, Volume 41, Number 6, 1319 “Stereotactic Transcranial Magnetic Stimulation: Correlation with Direct Electrical Cortical Stimulation”, wherein stereotactic transcranial magnetic stimulation (TMS) is used to pre-operatively functionally map the motor cortex. In this process, a patient's head is attached firmly and immovably to a headrest provided with a rotating arm, on which a figure-eight coil is arranged such that the tip of the arm lies beneath the intersecting point of the coil. In this way, the arm is aligned such that the tip lying beneath the intersecting point of the two coils points to a specific area in which a current is to be induced.